A microporous film is applied in various fields. Among the microporous films, a thermoplastic resin microporous film is widely applied in various fields, such as a separation membrane used for filtration or separation for a medical or industrial use, a hygienic material such as a backsheet for a disposable diaper, and a building material such as a house wrap and a roofing base material. In particular, a polyolefin resin microporous film has resistance to an organic solvent or an alkaline or acidic solution, and therefore is widely and suitably applied in the fields.
As an industrial process for manufacturing the polyolefin resin microporous film, two processes, namely, a phase separation process (wet process) and a stretching process (dry process) are generally well known.
In the wet process, the microporous film is formed by mixing a polymer and a solvent at a high temperature to prepare a homogeneous solution, processing the homogeneous solution into a film according to a T-die process, an inflation process or the like, and extracting and removing the solvent using a different volatile solvent, and then stretching the resultant film.
In the wet process, a porous structure can be controlled depending on a way of combining the polymer and the solvent, and according to diversified stretching methods such as uniaxial stretching by roll stretching, successive biaxial stretching by roll stretching and tenter stretching, and simultaneous biaxial stretching by simultaneous biaxial tenter stretching, or a processing method as in a case where stretching is carried out in a state of containing the solvent before extraction, and a case where stretching is carried out after removing the solvent.
However, the wet process involves essential problems due to use of a large amount of solvent, such as a heavy environmental load and a high manufacturing cost.
In the dry process, a crystal structure having a high regularity is formed by extruding a melted polymer from a T-die or a circular die, processing an extruded polymer into a film at a high draft ratio, and then applying heat treatment. The dry process includes a method for forming a porous structure by subsequently applying cold stretching and further hot stretching to the film to delaminate a crystal interface and produce a pore space part between lamellas (hereinafter, referred to as “single-ingredient stretching process”), and a method for generating voids (pores) in an interface between different kinds of polymers by mixing polyethylene, polypropylene and so forth to mold a sheet, and stretching the sheet at least in one direction. The former method is disclosed in Patent literatures Nos. 1 to 5 and so forth.